[vlc-devel] [PATCH 0/1] dshow: Fix assertion when stopping

Rémi Denis-Courmont remi at remlab.net
Wed May 31 18:26:56 CEST 2017

Le keskiviikkona 31. toukokuuta 2017, 9.52.26 EEST Hugo Beauzée-Luyssen a 
écrit :
> > In general, a plugin _cannot_ assume that its callbacks are in a single
> > thread.
> That seems like even more of a reason to go away from

Yes, of course. I didn´t mean to challenge that aspect.

> If we can't guarantee the thread from which
> things will be called, we either use MTA and deal with synchronisation
> if needed, or marshall all of our calls to COM functions. The later
> doesn't sound appealing to me.

TBH, I am not familiar with marshalling.

> > Isn´t FindDevices() potentially called from the GUI thread?
> > Isn´t the GUI thread entered in an STA?
> Only the Qt GUI calls CoInitializeEx (as far as interfaces are
> concerned, obviously)

Yes but isn´t the Qt Win32 port also entering an apartment internally, and isn
´t that unfortunately an STA?

> meaning the other interfaces would eventually enter an
> MTA, should they invoke FindDevices.

FindDevices() has to release whichever apartment that it succesfully entered. 
Otherwise we would have no way to prevent leakage.

> They would also leave the MTA after fetching the devices.
> > > As such, it is safe to create & release those objects from another
> > > thread without synchronisation.
> > 
> > As far as I know, it would be safe if and only if the given thread were
> > in the
> > MTA when it creates, uses or destroys any of the objects, *and* if the
> > MTA has
> > at least one outstanding reference throughout the lifetime of each
> > object.
> That is the case here. All objects are locally created (within
> FindCaptureDevice and AppendAudioEnabledVDevs), and guaranteed to be
> destroyed when leaving those function, which is before we call
> CoUninitialize

The enumeration callbacks indeed seem fine. But what about the access 
callbacks? What happens if Open() and Read(), or Open() and Close() are 
serialized yet called in different threads?

> Given that you say it would be safe if (...), and that this condition is
> met, I'm not sure why this patch is wrong. What would be the correct
> solution in your opinion?

AFAICT, either the DirectShow code has a thread keeping the MTA alive for the 
lifetime of the access/access_demux instance, or we ensure that another thread 
takes care of the same (e.g. in Win32 specific core code).

MMdevice has a background thread taking care of that, so the problem might 
only be apparent if you disable audio. The SAPI plugin is also buggy in this 
respect, IIRC.


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